Because of its structure, the play has no clearly defined protagonist or antagonist. However, some of the many characters are more obviously "good" or "bad" based on their decisions and motivations. Here is one possibility:
The Girl is the protagonist. Although she doesn't take a center role, she is the only character who is undeniably "good" in her actions. She is sympathetic and understanding, and does not do anything to harm or injure the other guests. She also takes time to try and help others, especially Paul, who is looking for his grandfather, and Morse, who is old and not entirely sane. She does tell Jackie that the purchased farmland is worthless, but is immediately ashamed of her outburst. While all the other characters are looking out for themselves, The Girl takes her time to look out for others.
Jackie is the antagonist. While she starts out with the laudable notion of helping her brother in his life, she is rude to everyone she meets, and it comes out that she stole a great deal of jewelry from Morse.
MORSE: My things! My wedding cuff links and my necklace that belonged to my wife!
MORSE: And four rings; a gold ring and a sapphire ring and -- I don't remember...
JACKIE: (Overlapping from "sapphire.") I got things to do -- you can search my car and you can search my room -- I've got nothing to hide; we got business.
(Wilson, The Hot L Baltimore, Google Books)
Because of Morse's apparent senility, it is possible that Jackie is telling the truth, but she further implicates herself by fleeing without word to others. Most tellingly, she abandons her brother Jamie, who is not mentally equipped to live on his own. Jackie is the character whose major actions hurt others, and acts in an entirely selfish manner.